The present invention concerns a novel circulating fluidized bed reactor.
A xe2x80x9cCirculating Fluidized Bed (CFB) Reactorxe2x80x9d stands for a multiphase flow apparatus formed by gas distribution nozzles, a riser i.e. a riser tube, a cyclone separator and a straight, unregulated return channel, a substantial part of the particles being conducted through the cyclone and returned from the cyclone to the lower part of the riser.
In the first CFB reactors, conventional cyclones of the type having a single inlet were used and the return channel of the solid material comprised a complicated construction with a gas lock, whereby the channel were fitted on the outside of the reaction chamber which made the construction expensive, clumsy as regards flow dynamics, and voluminous. When used, in particular, for steam boilers, this construction was associated with great constructional problems and, as a result of these problems, CFB reactors were subjected to active development work only in the time period after 1980. Since steam boilers were the commercially most interesting applications of CFB reactors, the development work was concentrated on constructions which would remove the known problems of the CFB boilers even with the risk of providing new constructions which, as far as process engineering was concerned, were of inferiour quality. The constructions created as a result of this thinking can be called second generation CFB reactors. Horizontal CFB boilers having quadrical cross section and employing cooled multi-inlet cyclones can, e.g., be mentioned as examples of second generation CFB reactors. All of these had in common that the whole construction was cooled and the protective materials were primarily used only for protection against wear. In small boilers also other part of the boiler had to be provided with insulation masses in order to prevent excessive cooling.
The second generation CFB reactors were poorly applicable to any other use than as steam boilers. In the process, chemical and oil refining industry the reactors are expected to meet requirements on reaction, heat and flow technique rather than constructional technical features. As a result, a new phase of the development work was initiated in the 1990""s with the aim of providing multi-inlet, concentrical CFB reactor technology with numerous applications. Since the starting point of the this development phase was formed by the second generation CFB reactors, it is possible to talk about already the third generation CFB reactors. The third generation CFB reactors are characterized by a multi-inlet cyclone fitted concentrically inside the riser.
Not even the third generation CFB reactors were throughout applicable to new applications, such as CFB dryers based on indirect heat transfer. If a multi-inlet cycklone is fitted on the heat transfer part, in accordance with the principles of the third generation solutions, e.g. the following problems occur:
The cyclone portion increases unnecessarily the height of the dryer
In the conical part of the cyclone a rather large pressure drop is generated which is detrimental to the function of the return channel since bed material is concentrated therein
In the middle of the heat exchanger a return pipe has to be fitted which has a rather big diameter and which reduces the heat transfer surface and is disadvantageous as regards mechanical strength properties
The outer surface of the heat exchanger part cannot be utilized for heat transfer
General problems of the third generation CFB reactors are, e.g. that the return channel and the cyclone located inside the reaction chamber are subjected to erosion and, at high reactor temperatures, also to corrosion. Further, the location of the cyclone and the return channel inside the riser chamber reduce mixing in horizontal direction. In several CFB applications (e.g. multistage regenerative heat exchangers) the reaction chamber height and pressure losses thereof become problematically large, because the minimum height is determined by the dimensions of the cyclone and the return channel. In this respect the third generation reactors are considerably much better than the earlier but even these encounter the above limitation which causes practical problems.
It is an aim of the present invention to eliminate the draw backs of the known technology and to provide an entirely novel circulating fluidized bed construction. In particular, it is an aim of the invention to provide a novel CFB reactor in which the height-to-diameter ratio can be equal to or smaller than 1.
The circulating fluidized bed reactor according to the invention generally comprises an elongated riser which has an at least essentially vertical central axis, feed nozzles fitted at the lower part of the riser for feeding solid matter and fluidization gas into the riser and a separation unit fitted at the upper part of the riser for separation of solids from the fluidization gas. The separation unit preferably consists of a multi-inlet cyclone, comprising a separation chamber with guide vanes for bringing the gas which is being treated into a gas flow directed along the inner surface of the separation chamber for separating the solid matter from the gas mainly under the influence of centrifugal force and a dipleg (returm channel) for recovering the solids.
According to the present invention the multi-inlet cyclone and the return channel are fitted annularly outside the riser. In this way a fourth generation CFB reactor is provided which is both fuctionally and constructively entirely different from the third generation CFB reactors.
More specifically, the CFB reactor according to the present invention is characterized by what is stated in the characterizing part of claim 1.
The invention provides considerable advantages. Thus, its most central benefits in comparison to all prior CFB reactor types is that it removes the earlier limitation that the ratio of the height to the diameter should be in practice greater than 1. In the device according to the invention said ratio can be 1 or smaller, e.g. about 0.1 to 0.95.
The flow dynamics of the present solution are beneficial because both the friction of the gas and the centrifugal force drive the particles obtained from the riser channel towards the wall of the cyclone.
If two separate interconnected CFB are used (e.g. a reactor and a regenerator), catalyst exchange between the reactors is easier to accomplish with fourth generation CFB reactors. The use of two separate CFB reactors may be more advantageous that the use of a nested construction when the amount of solids which should be changed is small in comparison to the amount of internal recirculation of the reactors.